Home › Imperial Fine (UNF) › 1/2-20
1/2-20 UNF bolt size
3/4 in wrench
across-flats spanner / socket size for 1/2-20
1/2-20 UNF bolt — automotive and machinery.
Full specification
| System | Imperial UNF |
| Nominal diameter | 1/2 in (12.7 mm) |
| Thread pitch | 20 TPI (fine) |
| Wrench / spanner (across flats) | 3/4 in |
| Tap drill | 29/64 in |
| Clearance hole | 33/64 in |
| Common grade / class | Grade 5 |
| Typical torque | 115 Nm (85 ft·lb) |
| Closest other-system size | ~1/2-13 coarse |
🔧 Torque unit converter
Pre-filled with the typical torque for 1/2-20 (Grade 5, dry). Edit any field to convert.
⚠️ Torque values here are general references for Grade 5 on clean, dry threads. Lubrication, coatings, and the joint material change the figure significantly. For structural, automotive, or safety-critical work, always use the torque specified by the manufacturer or engineer.
About 1/2-20
1/2-20 fine thread is widely used in automotive driveline parts.
A 1/2-20 nut will not start on a 1/2-13 bolt — confirm the pitch first.
Frequently asked questions
- What size wrench or spanner fits 1/2-20?
- 1/2-20 UNF bolt uses a 3/4 in wrench or spanner, measured across the flats of the hex head.
- What is the torque for 1/2-20?
- A typical tightening torque for 1/2-20 UNF bolt in Grade 5 on dry threads is about 115 Nm (85 ft·lb). Always confirm against the manufacturer spec for important joints.
- What thread pitch and tap drill does 1/2-20 use?
- 1/2-20 UNF bolt has a thread pitch of 20 TPI (fine) and uses a 29/64 in tap drill, with a 33/64 in clearance hole.